Electrical stimulation therapy is a fast-growing field, largely related to the successful use of implantable electrical stimulation devices for a wide range of applications. One important application is Deep Brain Stimulation (DBS). The DBS system may comprise two components: an implanted pulse generator (IPG) and a probe. The IPG is a battery powered neurostimulator that sends electrical pulses to the brain to interfere with neural activity at a target site. The probe typically consists of about 10-15 cm long wires and a plurality of electrodes. The wires connect the IPG to the electrodes, which are located at the distal end of the probe. The IPG may be calibrated by a neurologist, nurse or trained technician to optimize symptom suppression and control side effects.
Generally, however, there is a concern with implantable electrically conducting structures. It is desirable to be able to conduct Magnetic Resonance Imaging (MRI) on individuals having an implanted device. But due to the large changing magnetic fields associated with MRI, strong electrical fields can result and the associated current flow may heat up the tissue surrounding the conductor, which can damage the tissue. The problem is particularly found at conductor ends. For example, it has been shown that for an insulated 20 cm straight wire in brain tissue, the temperature may rise up to 48° C. in the normal operation mode of a 1.5 T MR systems. A temperature rise of less than 1° C. is considered safe.
The induced current densities in the electrodes and thus the undesired heating can be reduced by using electrodes with high impedances. The application of high impedance is however in conflict with having a long battery lifetime insofar the high impedance is realized as a high DC-resistance.
The published US patent application no. 2008/243218 discloses implantable leads with one or multiple conductors which from the configuration of the conductor(s) can reduce unwanted coupling to electrical field induced from the MR scanning. It is disclosed to configure the leads so they include at least one conductor with opposing proximal and distal portions, where the conductor turns back on it-self at least twice. The backward routing of the conductor, however, increases the length of the conductor.
The inventors of the present invention has appreciated that there is still a need in the art for improved implantable devices, and have in consequence devised the present invention.